The function of a polarizing beamsplitter (PBS) is to reflect light in one polarization state and to transmit light in the orthogonal polarization state. Consequently, PBSs find widespread use in optical systems that rely on the polarization of the light. An example of one such system is an image projection system that uses a liquid crystal display (LCD) panel as an “imager” for modulating an illumination light beam. A polarized illumination light beam is directed to the LCD panel and the light beam is spatially modulated by the LCD panel so that the beam contains some unmodulated light in the polarization state of the illumination beam and some modulated light having a polarization state orthogonal to the illumination light beam. The unmodulated, non-image light is reflected by the PBS and the modulated, image light, which contains the desired image, is transmitted through the PBS. Thus, the PBS separates the image light from the non-image light and the image light can then be projected to a screen for viewing by a user.
Different types of PBS may be used: projection systems have been reported using MacNeille PBSs, which rely on a stack of quarter wave films of isotropic material oriented at Brewer's angle for one of the polarization states, and using a Cartesian polarizer, including a wire grid PBS or a multilayer optical film (MOF) PBS, which uses a stack of alternating isotropic and birefringent polymer materials. The Cartesian MOF PBS is capable of operating at lower f-numbers and with higher contrast and transmission than the MacNeille PBS.
PBSs are often formed as a polarizing layer sandwiched between the hypotenuses of two glass prisms. The transmitted light is directed a different direction than the reflected light (for example at an angle of 90 degrees).
When a PBS is used in the optical path prior to arriving at an imager, the light reflected by the polarizer can usually be reflected back toward the light source. However, in the case of a transmissive imager, once light has passed through the imager, light reflected by subsequent polarizers cannot be allowed to be reflect back through the imager. Light reflected by these polarizers must be absorbed elsewhere in the projector so as to not reach the lens of the projector and detrimentally affect the image quality.
The polarizers typically used in projection systems have usually been absorptive type polarizers. An absorptive polarizer absorbs the majority of the light energy that is not of the desired polarization. Such a polarizer can absorb a relatively large amount of energy and the temperature of the polarizer tends to rise significantly. In many situations it is necessary to provide considerable airflow at high speeds over the polarizer in order to maintain the polarizer temperature below acceptable limits. In some cases, the required airflow volume and/or speed can be quite high and the size and number of fans needed in the projection device can consume a significant amount of electrical power and space in the projection device.